It is well known that in the present state of the art several methods are available to the obtaining and leveling of colors, by means of which color shades can be obtained highly resembling those previously specified color shades. The color shades obtained during the leveling process, however, do not comply 100% with the required color shade as, during the final color adjustment step, it is started from a subjective comparison as when the color to be obtained is compared to a color taken as a reference. Said comparison is effected by employing the criterium of a person skilled in color leveling or by employing some computerized system for color comparison.
On the other side, in the graphic arts industry several printing methods are known among which a plain colors printing method is common as well as a process colors printing method.
Printing through plain colors requires solid color opaque inks, each resulting from a mixture of different pigments, thus obtaining a diversity of colors; while the printing method through process colors requires of four pigments of standard transparent inks, namely, yellow, magenta, cyan and black.
In the printing method through process colors, small points or half-tone screens are generated, so as to form a printing mesh for each of the colors, and said meshes are arranged over one another.
Both in the plain colors printing method and in the process colors printing method, obtaining and leveling the color shades is a subjective problem, as in both methods it is started from a previously printed reference in order to define the required color; but this means not that the color obtained at the end of the operation be accurately the selected one.
In case of plain color printing, when a slightly clearer color is desired, white pigment is added; while for a darker shade black pigment or one or more dark pigments are added.
When the printing is already in press, clearer shades can be obtained in solid plain color inks, by employing a point mesh or screen on the paper, thus allowing that a bit more of the papel whitness appears.
As far as the process colors printing concerns, the various standard ink combinations of process colors (yellow, magenta, cyan and black), when used in different percentages of point sizes or screens, provide for the creation of an entire range of colors constituting the color gradation that can be obtained through the use of said printing method.
During the printing method through process colors, a press spreads on the paper the transparent yellow, magenta, cyan and black inks, as small points or half-tone screens, each of which varies in size from 1 up to 99 percent. In general, the following sequential order of application is employed during the printing: black, cyan, magenta and yellow.
It is important to mention that the color range to be obtained by means of the combination of said four standard process colors is more limited than the color range obtained with the ink solid colors; therefore, when the process colors printing is used, always will result in colors which are far away from many of the ink solid colors. This implies that only a simulation of many of the ink solid colors is possible with the use of the four process colors; and this, necessarily, ends with an unsatisfaction for the end user.
Presently those skilled in standard process colors carry out the color selection by employing color charts with samples of solid colors; which, as already mentioned, results in a simulation of the required color shade; whereby the color
Presently those skilled in standard process colors carry out the color selection by employing color charts with samples of solid colors; which, as already mentioned, results in a simulation of the required color shade; whereby the color obtained when printing with process colors does not suitably satisfy the requirements of the end user.
In the prior art exists several color separation processes for determining the proportion of the different inks, namely cyan, magenta, yellow and black to be used in the printing process, wherein an amount of black ink substitutes a partial amount of each of the color inks used.
International Publication No. WO 85/03581 discloses a four color printing process which uses three subtractive primary colors and black.
The process described in this document established that during the color separation phase for printing, the substractive primary color of least intensity in each printing area is totally removed and compensated by black, whereby each printing area contains a maximum of two primary colors and black; however, this process has the disadvantage of requiring the existence of experimental values for determining the color inks and black ink proportions.
U.S. Pat. No. 4,908,712 discloses a method for reproduction of a multi-tone color image by combination of chromatic inks of cyan, magenta and yellow together with an achromatic ink of black. In the region of lower tone levels than a prescribed value, the image is formed by the complete under color removal (UCR), namely, by the sole use of the achromatic ink. In the region of higher tone levels than the prescribed value, the reproduction of the image is attained by performing the under color removal (UCR) at a ratio of decrease relative to the rise of the tone levels, namely by causing the chromatic inks to overlap the achromatic ink.
This U.S. Patent also has the disadvantage of requiring the existence of experimental values for determining the color inks and black ink proportions.
On the other hand, International Publication No. WO 95/27923 describes a color separation process wherein the color ink proportions and the black portion are determined without the necessity of having experimental values.
For the purpose, the invention proposes a color separation process wherein the proportions of the colors (additive mixture) are determined for each image point of the original; the color proportions of the original are converted into proportions of process inks (subtractive mixture); the value for the smallest process ink proportion is set as the value for the black portion, representing the amount of black for reproducing the image point of the original; based on the black portion the residual white portion of the image point is determined; differential process ink proportions are determined based on the difference of the previously determined process ink proportions and the black portion; the differential process ink proportions are converted to corrected process ink proportions representing the amounts of the process inks needed to reproduce the image point, the corrected process ink proportions then being equivalent to the process ink proportions which, starting from the differential process ink proportions, correspond to process ink proportions for reproduction on a completely white surface.
Although this document states that the ink proportions and the black portion are determined without necessity of having experimental values, by means of the method described therein, it is impossible to obtain a color shade without doing a color separation first, which strictly is an experimental procedure. Therefore, it is impossible also to obtain an intermediate shade in a color chart due to the fact that the color separation cannot be effected since the color shade is not shown.
As a consequence from the above, a way to supress the drawbacks of the prior art and provide a method to obtain a tabulation has been sought, by means of which an accurately reproductible color range can be represented; thus providing the necessary tools to carry out a color selection in a simple and practical manner, such as a catalogue or color chart; and also allowing, during the application of any of the printing methods, the accurate reproduction of said colors; or else, usable in any application requiring the selection and procuration of colors to be accurately reproduced.